A Thai court on Wednesday sentenced a man to 20 years in prison for sending text messages deemed insulting to the monarchy, his lawyer said, under the kingdom's strict lese-majeste laws.

Ampon Tangnoppakul, 61, was found guilty of four counts of sending messages to the private secretary of then-prime minister Abhisit Vejjajiva in May 2010, according to the criminal court in Bangkok.

"The court found him guilty and sentenced him to 20 years in jail," his lawyer Anon Nampa told AFP, adding that he had 30 days to lodge an appeal.

Ampon was arrested in August last year and pleaded not guilty to the charges during his trial.

After his arrest, Thailand's Central Bureau of Investigation said the messages were "inappropriate and considered insulting to the monarchy and have upset the recipients," without revealing their content.

The royal family is a very sensitive subject in Thailand. King Bhumibol Adulyadej, 83, is the world's longest-reigning monarch and revered as a demi-god by many Thais.

Under Thai law, anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

Academics have noted a sharp increase in new royal insult cases in recent years and rights groups have expressed concern that the law was being used to suppress freedom of expression under the previous government.

Last month a Thai-born US citizen pleaded guilty to insulting the monarchy.

Joe Wichai Commart Gordon was arrested in May during a holiday in the kingdom and accused of posting a link to a translation of a banned book on his blog, along with other material deemed offensive, while living in the United States.

In the wake of Gordon's case, the UN special rapporteur on freedom of expression, Frank La Rue, urged Thailand to amend the lese-majeste laws, saying they stifled debate on matters of important public interest.

The UN call prompted the Thai foreign ministry to accept the laws may have been misused and stress their aim was to protect the dignity of the monarchy, not restrict free speech.

The ministry said a special committee in the Royal Thai Police headquarters had been set up to scrutinise potential prosecutions.

A prominent Thai website editor is also facing up to 20 years in jail on charges relating to remarks about the monarchy posted by other people on her website.

Chiranuch Premchaiporn, the editor of the popular Prachatai news website, denies allegations that she did not remove 10 online posts perceived as critical to the monarchy quickly enough in 2008.